1. Field of the Invention
The invention relates to a radiation-source unit for producing a radiation beam having two components which are polarised perpendicularly to one another and which have different frequencies, which unit comprises a coherent radiation source, a beam splitter, an acousto-optical modulation system for generating a frequency difference between two sub-beams formed by the beam splitter, and a beam combiner for recombining the two sub-beams emerging from the modulation system to one beam.
The invention also relates to an interferometer and to a device for detecting the position of two objects relative to one another, both provided with such a radiation-source unit. The invention further relates to an apparatus for projecting a mask onto a substrate, comprising such an interferometer and/or such a position-detection device.
2. Discussion of the Related Art
The article "Displacement measurement with a laser interferometer" in "Philips' Technical Review", Vol. 30 (1969), pp. 160-166, describes an interferometric displacement measuring device whose radiation-source unit is a so-called Zeeman laser. Such a laser comprises, for example, a single-mode He-Ne laser, across which a magnetic field is applied in the longitudinal direction. As a result, the laser generates two opposite circularly polarised modes having different optical frequencies instead of a single linearly polarised mode.
However, a Zeeman laser can deliver only a limited power, which is particularly disadvantageous in more recent applications in which the laser beam is to be split into more than two beams. Moreover, a Zeeman laser is comparatively expensive and very sensitive to optical feedback, which means that radiation reflected into the laser from the optical measuring system may cause variations in amplitude and frequency of the emitted laser beam, which may affect the measurement result. In addition, the frequency difference between the two mutually perpendicularly polarised components is at the most 2 MHz, so that the Zeeman laser is suitable only for measuring limited distances or limited speeds.
U.S. Pat. No. 5,191,465 describes a radiation-source unit specially intended for aligning a mask relative to a substrate in an opto-lithographic apparatus for imaging the mask on the substrate. This radiation-source unit comprises a laser, a neutral beam splitter, a separate acousto-optical modulator in each of the paths of the sub-beams formed by the beam splitter, and a polarisation-sensitive beam combiner. The drive signals for the two modulators have different frequencies, so that different frequencies are impressed upon the sub-beams. A .lambda./2 plate has been arranged in the path of one of the sub-beams, as result of which the two sub-beams have mutually perpendicular directions of polarisation. As both the beam splitter and the beam combiner are semitransparent reflectors and two further reflectors have been provided the radiation-source unit in accordance with U.S. Pat. No. 5,191,465 is highly susceptible to alignment errors and its stability is unsatisfactory.